Advances in plasma processing have provided for growth in the semiconductor industry. In today competitive market, a manufacturing company needs to be able to minimize waste and produce high quality semiconductor devices. Accordingly, tight control of the process parameters is generally needed to achieve satisfactory results during substrate processing.
Within a processing chamber, gas may interact with a radio frequency (RF) current to form plasma during substrate processing. In order to control plasma formation and to protect the processing chamber walls, the plasma may be confined to a limited chamber volume, such as the region within a peripheral ring. To exhaust the neutral gas species from the confinement region (the volume within the peripheral ring), the peripheral ring may include a plurality of slots. Each slot has a geometry that is configured to be large enough to allow the neutral gas species to exit the confinement region and flow toward the turbo pump by traversing through the slots. Generally, to be effective in confining the plasma within the confinement area, each slot tends to have a cross-sectional of less than two times the plasma sheath. As discussed herein, plasma sheath can exist on each side of the slots. Hence if the total sheath thickness greater than the slot width, there won't be any bulk plasma in between the sheath hence plasma successfully pinch of by slots. However if the slot width greater than the tow times the sheath thickness then the plasma can exist inside the slots.
Those skilled in the art are aware that each recipe/recipe step may require a certain pressure volume/level to be maintained in order to generate the desired plasma needed during substrate processing. However, during substrate processing, certain conditions (e.g., chamber conditions) may cause the pressure volume/level to fluctuate. To control the pressure volume/level, a vacuum valve that is positioned downstream from the confinement region but upstream from the turbo pump may be employed. In an example, to increase the pressure volume/level, the vacuum valve may be tightened.
Unfortunately, as the pressure increases, the plasma sheath tends to collapse and the size of the plasma sheath may become smaller. In some circumstances, the cross-sectional size of each slot may become larger than twice the size of the shrinking plasma sheath. Given that the plasma sheath has shrunk, the slots may no longer be able to confine the plasma within the confinement region. As a result, plasma may traverse through the slots and be formed outside of the confinement region. This is especially true given that the tightening of the vacuum valve not only increases the pressure volume/level within the confinement region but also the pressure volume/level in the outside chamber volume (the region outside of the confinement region). Thus, the high-pressurized environment of the outside chamber volume may be conducive for unconfined plasma formation.
Accordingly, an arrangement for pressure control while restricting plasma formation within the region formed by the peripheral ring is desirable.